1. Technical Field
The present invention relates to methods of improved enzymatic hydrolysis of cellulosic biomass by addition of hydrothermally treated stillage or a fraction thereof derived from a dry-grind ethanol process.
2. Background Art
Ethanol is an important component of liquid motor fuel supply. It is used as an oxygenate, octane booster and extender in gasoline. It is typically blended with gasoline to a concentration of 10% (E-10) or 85% (E-85). In the United States, ethanol is primarily made from sugars derived from corn starch. Cellulosic materials, such as wheat straw, grasses, corn stover (the leaves and stalks that remain after corn harvest) and corn cobs, silage, wood chips, and forest trimmings, are now being used as an alternative to corn starch to produce ethanol. Additionally, corn kernel fiber is also being considered as a feedstock. Cellulosic material is more abundant than corn and ethanol produced from such material has a greater greenhouse reduction. The EPA has called for 16 billion gallons of ethanol to be produced from cellulosic sugars by 2022.
Cellulosic materials are composed of carbohydrate polymers (celluloses) and aromatic polymers (lignin). The sugars of the carbohydrate polymers are tightly bound to the lignin. In the production of ethanol, the sugars are necessary for fermentation and must be extracted from the lignin.
One method of extracting these sugars in the cellulosic materials is enzymatic hydrolysis. While this is currently the most promising technology, it is not cost effective. Enzymatic hydrolysis requires high doses of expensive enzymes and produces relatively low sugar yields. While costs of enzymes have come down over the years, it remains a first order cost.
Methods have been employed to improve the economics of biomass enzymatic hydrolysis. A major improvement in the process economics can be realized by the effective pre-treatment of the cellulosic materials to dissolve or disrupt the lignin structure. This improves enzyme access to the cellulosic components and enhances enzymatic activity. Many pre-treatment methods have been investigated and have been extensively reviewed in the literature (V. B. Agbor et al./Biotechnology Advances 29 (2011) 675-685). Pre-treatment methods include liquid hot water, steam explosion, ammonia fiber expansion (AFEX), aqueous ammonia treatment, ionic liquid fractionation, alkali treatment, ionic liquids, OrganoSolv process, dilute acid treatment, alkaline peroxide treatment, wet air oxidation, and low temperature steep delignification.
Other efforts to improve the economics of enzymatic hydrolysis focus on reduction and recycle of the enzymes.
U.S. Pat. No. 8,367,378 to Balan, et al. (assigned to Michigan State University) discloses a process for improving the enzymatic hydrolysis of ammonia pre-treated biomass by the addition of thin stillage or a clarified liquid portion of thin stillage. Balan, et al. does not teach or suggest improved enzymatic hydrolysis by the addition of clarified, hydrothermally treated stillage to pre-treated cellulosic biomass.
U.S. Patent Application Publication No. 2012/0244591 to T. Brotherson (assigned to Quad County Corn Processors) discloses a process to produce ethanol from the cellulose and hemicellulose present in the fiber fraction of whole stillage. The stillage is heated and acidified as a pre-treatment prior to addition of hydrolyzing enzymes. This application neither teaches nor suggests any benefit of stillage to the enzymatic hydrolysis step but is rather focused on utilization of the fiber in stillage as an additional source of sugars for ethanol production.
PCT/US2012/021731 to Narendranath, et al. (assigned to POET LLC) discloses a process for treating cellulosic biomass including the addition of a clarified thin stillage stream to the enzymatic hydrolysis step. Narendranath, et al. disclose that addition of either a thin stillage composition or anaerobic digestion effluent, each having substantially reduced insoluble solids, to the enzymatic hydrolysis step improves release of sugar from pre-treated cellulosic biomass and further improves fermentation of the hydrolysate to ethanol. Narendranath, et al. do not teach or suggest improved enzymatic hydrolysis by the addition of clarified, hydrothermally treated stillage to pre-treated cellulosic biomass.
U.S. Patent Application Publication No. 2010/0159552 to R. Benson and R. Benech (assigned to Greenfield Ethanol) discloses a process for improved fermentation of pre-treated cellulosic biomass by addition of a stillage residue stream. Benson and Benech do not teach or suggest the addition of a stillage residue stream to the enzymatic hydrolysis step; nor do they teach or suggest that hydrothermal treatment of the stillage stream affords an improved hydrolysis media.
There remains a need for a cost effective method of enzymatic hydrolysis of cellulosic materials in order to produce ethanol from cellulosic material.